The present disclosure generally relates to casting molds for use in directional solidification casting processes and methods of making the same. More particularly, the casting mold includes a graded facecoat structure that is configured to prevent or delay a liquid metal cooling agent from reacting with a surface of the casting metal during the directional solidification process.
Directional solidification processes are methods commonly employed for producing parts such as turbine blades and vanes with columnar and single crystal growth structures. It is well known that the high temperature mechanical properties of single crystal superalloys are superior to castings with polycrystalline structures. Generally, a desired single crystal growth structure is created at the base of a vertically disposed mold defining a part. Then, a single crystal solidification front is propagated through the structure under the influence of a moving thermal gradient.
Various processes exist for directional solidification. In one process, directional solidification can be achieved by slowly lowering a mold containing molten metal out of a heated furnace and into a liquid cooling bath, which acts as a cooling medium for the molten material. The cooling bath can include a metal that is heated to a temperature below the melting point of the molten material and above the melting point of the liquid cooling agent. Solidification of the molten material can progress from the bottom to the top of the mold as the mold is lowered into the cooling bath. In this manner, a solid-liquid interface can advance upward as heat is transferred from the molten material within the mold to the liquid cooling bath. One of the problems of liquid cooling the mold is the propensity that the liquid cooling agent contaminates the casting material, resulting in undesirable surface pitting. Reaction of the liquid cooling agent with the casting metal can occur if the mold is not properly sealed or if it cracks prematurely before completion of a casting run.
Facecoats are sometimes used to form a protective barrier between the molten casting metal and the surface of the shell mold. For example, U.S. Pat. No. 6,676,381 (Subramanian et al.) describes a facecoat based on yttria or at least one rare earth metal and other inorganic components, such as oxides, silicides, silicates, and sulfides. The facecoat compositions are most often in the form of slurries, which generally include a binder material along with a refractory material such as the yttria component. When a molten reactive casting metal is delivered into the shell mold, the facecoat prevents the undesirable reaction between the casting metal and the walls of the mold, i.e., the walls underneath the facecoat. Facecoats can sometimes be used, for the same purpose, to protect the portion of a core (within the shell mold), which would normally come into contact with the casting metal. Current facecoats are prone to failure such as when the mold body cracks during the casting process.
Accordingly, it would be desirable to have a casting mold suitable for directional solidification processes that employ a liquid cooling bath, wherein the mold is configured to prevent or delay a liquid metal cooling agent from reacting with a surface of the casting metal during the directional solidification process.